In n-well bulk CMOS (complementary metal oxide semiconductors) the n-well transistor source and drains are implanted into the p-substrate, and p-transistors are implanted into the n-well. The n-well has been previously implanted into the p-substrate. The diffusion capacitance of both source and drain consists of the perimeter term plus the area term between the implants and the substrate.
For a silicon-on-insulator ("SOI") CMOS structure, a buried oxide lies under the transistor. The source, drain and bulk are bound by the buried oxide layer on the bottom, and the bulk (body) is floating. This topology substantially reduces the diffusion capacitance of the source and drain. Hence, the device may perform at much higher speeds and lower power. Furthermore, due to the device structure, soft error susceptibility is reduced, latch-up is eliminated, and short channel effects are improved such that the device may be designed with a lower threshold voltage, therefore further increasing the operational speed of the device.
As SOI CMOS technology becomes available, high speed processors may take advantage of the increase in performance. There are, however, several problems associated with SOI that must be overcome by circuit design techniques. One of the SOI related problems is the parasitic bipolar effect. FIG. 1 illustrates a cross-section of an NMOS SOI transistor 100. A buried oxide layer 102 is deposited on a substrate 101. A p-doped layer of silicon 103 is then deposited on the buried oxide 102 and used as the bulk layer of the transistor 100. Between the center p-bulk portion 103 and an oxide portion 107, an n+ source 106 is created, while an n+ drain portion 104 is created between p-bulk 103 and an oxide portion 105. On top of the p-bulk 103 portion, a gate oxide 108 is deposited and then a material for operating as the gate electrode 109 is deposited on top of the gate oxide layer 108.
The bulk 103 consisting of p material is floating, unlike bulk CMOS circuits where the p-bulk is tied to ground. If the gate of the SOI NMOS device 100 is grounded as indicated, the device 100 is off. Vdd is applied to the drain 104, and the source 106 is left floating. Should this condition persist for a long time, drain 104 to body 103 junction leakage current and impact ionization generation near the drain gate region will eventually charge the bulk 103 to Vdd, and the source 106 will also float to Vdd over time. If switch S1 is closed, the source 106 is suddenly pulled to ground. This forward biases the bulk/source junction. As a consequence, the parasitic bipolar transistor consisting of the drain 104 (collector), bulk 103 (base), and source 106 (emitter) is turned on and current flows through the transistor 100 even though the gate 109 is tied to ground.
Referring next to FIG. 2, there is illustrated a prior art distributed output multiplexer 200 for a column of n bits in a register file, implemented in SOI CMOS. For simplicity, only a single column is shown; a general purpose register file may have 64 columns or more and as many as 128 or more rows.
The dynamic node 211 represents the bit line. A single write and single read port memory cell 250 is shown in detail, even though the number of read and write ports may be larger. The other illustrated memory cells 217 and 218 are only shown in block diagram form.
Only one row per port in the register file is accessed during a read. For instance, if the register file has 64 bits per row and 128 entries and 6 read ports, then between one and six rows (different ports may read from the same address) are read from during any read cycle. An individual row may be accessed to every cycle or not at all for a long time. However, it may be written to independently of the read operation.
The memory element consists of transistors 203-206. Memory cell0 250 is written to by issuing the write address wr0.sub.-- addr, which turns on pass transistors 201 and 202. True and complement write data is presented on the wrdata0 and wrdata0 inputs respectfully and passed into the memory cell 250 through pass transistors 201 and 202, respectively. After the differential write operation has been performed, the inverter consisting of PFET 207 and NFET 208 inverts the complement data, and if a 1 was written into the cell 250, then NFET 209 of the distributed output mux 200 is turned on.
Node 211 in distributed output mux 200 is precharged high when reset is low (see the timing diagram in FIG. 4), turning on PFET 223. Inverter 221 forces output dout low. This turns on transistor 222, which maintains the precharge condition for node 211 once the reset signal goes high. When at a later time the read address rd.sub.-- addr0 goes high, node 211 is pulled down and output dout goes high, provided that a 1 was present in the memory cell 250 from a previous write operation.
If a 0 was written into cell0 250, the pull down path through device 210 is blocked because device 209 has been turned off. When the read address goes high, node 211 remains high and output dout remains low. Devices 210 and 209 form the pull down mux of cell0, devices 215 and 216 represent the pull down mux for cell1, and devices 220 and 219 represent the nth cell pull down mux (all cells are in the same column). Only cell0.sub.-- column0 in conjunction with inverter 207, 208 and mux pull downs 210 and 209 are described. It should be understood that all cells in the array function identically.
For this discussion, the gate electrodes of devices 210 and 209 are both low, hence both transistors are off (0 has been written into cell0), and node 211 is precharged to Vdd. Assume now that the cell data has not been written to 1 for a long period of time (500 picoseconds; see FIG. 4) and no read operation has been performed. The bulk of device 210 becomes slowly charged to Vdd, and node 212 drifts up to the level of Vdd. If device 209 is suddenly turned on when a 1 is written into the memory cell0 (see FIG. 4), it will pull node 212 to ground. As a consequence, the bulk to source pn junction in device 210 is forward biased turning on the parasitic bipolar transistor of device 210 consisting of the drain (collector), bulk (base), and source (emitter). As a result current is flowing from the dynamic node 211 to ground.
A single parasitic bipolar transistor may be too weak to discharge the dynamic node 211. However, if a large number of ports are connected to node 211, all may exhibit the effects of the parasitic bipolar transistor simultaneously after being turned on in a single write cycle. The half latch 222 may now be too small to supply sufficient current to maintain the precharge state of node 211, and the output dout may rise to a level of 1. Even if the precharge state is maintained, the noise margin is substantially reduced. The result is additional power consumption which would be a major concern. Therefore, there is a need in the art for improved circuitry whereby parasitic bipolar effects are eliminated in an SOI circuit.